JP2012048122A - Image forming apparatus and image forming method - Google Patents

Abstract

An image forming apparatus that suppresses occurrence of abnormal discharge and suppresses deterioration in image quality.
An image forming apparatus includes a discharge wire and a grid, and includes a charger that charges a photosensitive member, a voltage application unit that applies a charging voltage to the charger, and a charging voltage that is CHG. A voltage detection unit 68 for detection and a control unit 51 that performs constant voltage control of the voltage application unit 60 based on the detection voltage D1 are provided. In addition, the image forming apparatus includes a neutralization unit that neutralizes the photoconductor 44, a current detection unit 72 that detects a current flowing from the charger 41 to the photoconductor 44 in a state of being neutralized by the neutralization unit, and a current detection unit 72. When the detected current detected is less than a predetermined current value, the control unit 51 includes a target value changing unit 51 that increases the target value of the charging voltage CHG for constant voltage control by a predetermined voltage amount.
[Selection] Figure 2

Description

  The present invention relates to an image forming apparatus and an image forming method, and more particularly to electrical control of a charger used in the image forming apparatus when an image is formed.

  As the electrical control of the charger, when charging the photosensitive member of the image forming apparatus, a technique for controlling the constant current so that the charging current flowing when the charging voltage is applied to the charger is constant is disclosed in, for example, a patent. It is disclosed in Document 1.

Japanese Patent Laid-Open No. 11-109715

  In the technique disclosed in Patent Document 1, a constant current can be passed through the photoconductor, and the image quality can be maintained. However, when the discharge wire of the charger becomes dirty, there is a possibility that the charging voltage increases in order to flow a constant current and abnormal discharge occurs. When abnormal discharge occurs, the image quality is degraded.

  On the other hand, if the constant voltage control is performed so that the charging voltage is constant, the constant voltage can be maintained, so that the occurrence of abnormal discharge can be suppressed. However, depending on the charged state of the photoconductor, current may not be sufficiently passed through the photoconductor, and the photoconductor cannot be sufficiently charged, and toner is deposited on an unexposed portion, leading to deterioration in image quality. There was a fear.

  The present invention provides a technique for suppressing the occurrence of abnormal discharge and suppressing deterioration in image quality.

  An image forming apparatus according to a first aspect of the present invention includes a photoconductor, a discharge wire and a grid, a charger for charging the photoconductor, a charging voltage, and the generated charging voltage applied to the charger. A voltage application unit that detects the charging voltage, a control unit that performs constant voltage control on the voltage application unit based on the detection voltage detected by the voltage detection unit, and neutralizes the photosensitive member. A neutralizing unit, a current detecting unit for detecting a current flowing from the charger to the photoconductor in a state of being neutralized by the neutralizing unit, and a detection current detected by the current detecting unit being less than a predetermined current value A target value changing unit for increasing the target value of the charging voltage for constant voltage control by the control unit by a predetermined voltage amount, and constant voltage control of the charging voltage at the target value changed by the target value changing unit. , And an image forming unit for forming an image on a recording medium.

  According to this configuration, it is possible to perform constant voltage control based on the charging current flowing through the photoconductor that has been neutralized, that is, based on substantially the maximum current required for charging the photoconductor. Therefore, even if black solid printing (one-sided black printing) that suppresses the occurrence of abnormal discharge and the charging potential of the photosensitive member decreases to the maximum, the photosensitive member can be sufficiently charged after that, so that subsequent printing A decrease in image quality can be suppressed.

According to a second aspect of the present invention, in the image forming apparatus of the first aspect, the target value changing unit increases the target value of the charging voltage by the predetermined voltage amount until the detected current becomes equal to or greater than the predetermined current value. repeat.
According to this configuration, since the target value of the charging voltage that can sufficiently charge the photoconductor can be set reliably, it is possible to reliably suppress the deterioration in image quality.

According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the current detection unit detects a current flowing through the photoconductor by detecting a current flowing through the grid of the charger.
According to this configuration, since it is not necessary to directly detect the current flowing to the photoconductor, deterioration of the surface of the photoconductor can be suppressed.

According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects of the invention, the static elimination unit partially neutralizes the surface of the photoconductor, and the target value changing unit includes: When the current detected when the neutralized surface portion of the photoconductor faces the charger, the target value of the charging voltage is increased by the predetermined voltage amount.
According to this configuration, for example, the non-image forming area in the longitudinal direction (axial direction) of the photosensitive member is not neutralized, and only the image forming area is neutralized, so that the photosensitive member can be sufficiently charged. And it can suppress that an excessive electric current flows.

A fifth invention is the image forming apparatus according to any one of the first to fourth inventions, wherein the target changing unit is detected by the voltage detecting unit when the detected current is less than the predetermined current value. If the charging voltage is less than a predetermined voltage value, the target value of the charging voltage is increased by the predetermined voltage amount.
According to this configuration, if the predetermined voltage value is set to a value that suppresses the occurrence of abnormal discharge, the occurrence of abnormal discharge can be more reliably suppressed.

According to a sixth aspect of the present invention, in the image forming apparatus of the fifth aspect, the control unit notifies the cleaning of the discharge wire if the charging voltage detected by the voltage detection unit is equal to or higher than the predetermined voltage value. .
According to this configuration, it is possible to improve the environment in which abnormal discharge is likely to occur by notifying the cleaning of the discharge wire.

According to a seventh aspect of the present invention, in the image forming apparatus according to any one of the first to sixth aspects, the exposure unit further includes an exposure unit that forms a charged latent image on the surface of the photosensitive member by exposing the photosensitive member. By exposing the predetermined surface area of the photoconductor uniformly with a predetermined light intensity, the exposure unit constitutes the charge eliminating unit.
According to this configuration, a dedicated device is not required and the surface of the photoconductor can be directly discharged as compared with a charge removal method using an EL lamp or a transfer current.

According to an eighth aspect of the present invention, in the image forming apparatus according to the seventh aspect of the present invention, the image forming apparatus further includes a developing unit that develops the charged latent image on the surface of the photoconductor. When doing so, the photosensitive member and the developing device are separated from each other.
According to this configuration, it is possible to suppress a large amount of developer from adhering to the static elimination portion.

According to a ninth invention, in the image forming apparatus according to any one of the first to eighth inventions, the image forming apparatus includes a plurality of sets of the photoconductor and the charger corresponding to each printing color, and the voltage application unit includes the charging voltage. Is commonly applied to a plurality of chargers.
According to this configuration, in the configuration in which a plurality of sets of photoreceptors and chargers are provided, the voltage application unit is shared, so that the number of parts that generate the charging voltage can be reduced.

A tenth aspect of the image forming apparatus according to the ninth aspect of the present invention has a print mode of a monochrome mode in which printing is performed using a black developer and a color mode in which printing is performed using a plurality of color developers. The target value changing unit sets a monochrome target voltage value of the charging voltage in the monochrome mode and a color target voltage value of the charging voltage in the color mode, and the charge eliminating unit sets the monochrome target voltage value. At the time of setting, only the photoconductor corresponding to black is neutralized, and at the time of setting the color target voltage value, the photoconductor corresponding to each color is neutralized, and the image forming unit is set at the target value of the charging voltage according to the mode. An image is formed on the recording medium under constant voltage control.
According to this configuration, in each of the monochrome mode and the color mode, an image can be formed with the optimum target value of the charging voltage, so that it is possible to favorably suppress deterioration in image quality.

According to an eleventh aspect of the invention, in the image forming apparatus according to any one of the first to tenth aspects, the target value changing unit increases the value of the predetermined voltage amount as the number of formed images increases.
According to this configuration, since the target value of the charging voltage can be set in consideration of the deterioration of the photosensitive member, it is possible to suitably suppress the deterioration of the image quality.

According to a twelfth aspect of the present invention, in the image forming apparatus according to any one of the first to eleventh aspects, the target value changing unit shortens the interval of the charging voltage target value changing process at high temperature and high humidity.
According to this configuration, it is possible to stably suppress deterioration in image quality even at high temperatures and high humidity.

  An image forming method according to a thirteenth aspect of the invention includes a photoreceptor, a discharge wire and a grid, a charger for charging the photoreceptor, and a voltage for applying a constant voltage controlled charging voltage to the charger. An image forming apparatus including an applying unit, wherein the image forming method includes a step of discharging the photoconductor, applying a predetermined voltage from the voltage applying unit to the charger, and removing the charge from the charger. Detecting a current flowing to the photoconductor in a performed state, and, if the detected current detected is less than a predetermined current value, increasing a target value of the charging voltage for constant voltage control by a predetermined voltage amount; Forming an image on a recording medium under constant voltage control of the charging voltage with the changed target value.

  According to the image forming apparatus and the image forming method of the present invention, it is possible to suppress the occurrence of abnormal discharge and to suppress the deterioration of image quality.

1 is a schematic cross-sectional view illustrating an internal configuration of a printer according to an embodiment of the invention. Schematic block diagram related to high-voltage power supply device for printer Flow chart showing each process of the printing process Flowchart showing processing relating to determination of color target charging voltage value Flow chart showing processing relating to determination of monochrome target charging voltage value

<Embodiment>
An embodiment 1 of the present invention will be described with reference to FIGS.

1. Overall Configuration of Printer FIG. 1 is a schematic cross-sectional view showing the internal configuration of a color printer 1 (an example of an image forming apparatus) according to the present embodiment. In the following description, when distinguishing each component for each color, subscripts of Y (yellow), M (magenta), C (cyan), and K (black) are added to the reference numerals of the respective parts, and they are not distinguished. Omits subscripts. Note that the image forming apparatus is not limited to a color printer, and may be, for example, a monochrome printer or a multifunction machine having a FAX and a copy function.

  A color printer (hereinafter simply referred to as “printer”) 1 includes a paper feeding unit 3, an image forming unit 5, a transport mechanism 7, a fixing unit 9, and a high-voltage power supply device 50. For example, the printer 1 generates a toner image composed of toner (developer) of one or a plurality of colors (four colors of yellow, magenta, cyan, and black in this embodiment) corresponding to image data input from the outside. Paper, OHP sheet, etc.).

  The sheet feeding unit 3 is provided at the lowermost part of the printer 1 and includes a tray 17 that accommodates sheets (an example of “recording medium”) 15 and a pickup roller 19. The sheets 15 accommodated in the tray 17 are taken out one by one by a pickup roller 19 and are sent to the transport mechanism 7 via the transport roller 11 and the registration roller 12.

  Further, a temperature / humidity sensor 36 that detects the temperature and humidity in the printer 1 and a paper sensor 37 that detects the number of conveyed sheets 15 are provided in the vicinity of the paper feeding unit 3. The paper sensor 37 is provided in the conveyance path of the sheet 15 taken out from the tray 17 by the pickup roller 19, and contacts the sheet 15 when the sheet 15 passes through the conveyance path provided with the paper sensor 37. Thus, a paper detection signal is output to the CPU 51 (see FIG. 2).

  The transport mechanism 7 is for transporting the sheet 15, and is detachably mounted on a predetermined mounting portion (not shown) formed in the printer 1, for example. The transport mechanism 7 includes a driving roller 31, a driven roller 32, and a belt 34, and the belt 34 is bridged between the driving roller 31 and the driven roller 32. When the drive roller 31 rotates, the surface of the belt 34 facing the photosensitive drum 44 moves from the right direction to the left direction in FIG. As a result, the sheet 15 sent from the registration roller 12 is conveyed below the image forming unit 5. The transport mechanism 7 includes four transfer rollers 33.

  The image forming unit 5 includes four process units 40Y, 40M, 40C, and 40K and four exposure apparatuses (an example of a charge removal unit). Each process unit 40 includes a charger 41, a photosensitive drum (an example of a photosensitive member) 44, a unit case 46, a developing roller 47, and a supply roller 48. Each process unit 40Y, 40M, 40C, 40K is detachably attached to a predetermined attachment portion (not shown) formed in the printer 1.

  The photosensitive drum 44 is formed, for example, by forming a positively chargeable photosensitive layer on an aluminum substrate, and the aluminum substrate is connected to the ground of the printer 1 via, for example, a conductive shaft 44a. Grounded to line. The charger 41 is, for example, a scorotron charger, and includes a discharge wire 42 and a grid 43. The charging voltage CHG is applied to the discharge wire 42, and the grid voltage GRID of the grid 43 is controlled so that the surface of the photosensitive drum 44 has substantially the same potential (for example, + 700V).

  The exposure device 45 includes, for example, a plurality of light emitting elements (for example, LEDs) arranged in a line along the rotational axis direction of the photosensitive drum 44, and the plurality of light emitting elements are selected according to image data input from the outside. By controlling the light emission, the surface of the photosensitive drum 44 is exposed. That is, the exposure device 45 attaches the toner by irradiating the photosensitive drum 44 charged to +700 V by the charger 41 with light to a portion where the toner for forming an image based on the image data is to be attached. The portion to be discharged is discharged, and an electrostatic latent image is formed on the surface of the photosensitive drum 44. The exposure device 45 is controlled to emit light with a light intensity that sets the potential of the exposed portion of the photosensitive drum 44 to, for example, + 200V. The exposure device 45 is fixedly installed in the printer 1. The exposure device 45 may use a laser.

The unit case 46 stores toner of each color (in this embodiment, for example, a positively chargeable non-magnetic one-component toner), and includes a developing roller 47 and a supply roller 48. The toner is supplied to the developing roller 47 by the rotation of the supply roller 48 and is positively frictionally charged between the supply roller 48 and the developing roller 47. Further, the developing roller 47 to which a developing voltage of +300 to 400 V is applied by the high voltage power supply device 50 supplies the toner onto the photosensitive drum 44 as a uniform thin layer, and attaches the toner to the exposed portion of the photosensitive drum 44. The electrostatic latent image is developed to form a toner image on the photosensitive drum 44.
Each developing roller 47 is connected to a separation mechanism 35 (see FIG. 2) for separating each developing roller 47 from each photosensitive drum 44.

  Each transfer roller 33 is disposed at a position where the belt 34 is sandwiched between each transfer drum 33. Each transfer roller 33 receives a toner image formed on the photosensitive drum 44 by applying a transfer voltage TRCC having a polarity opposite to the toner charging polarity (here, negative polarity) to the photosensitive drum 44. Transfer to the sheet 15. Thereafter, the sheet 15 is conveyed to the fixing unit 9 by the conveying mechanism 7, and the toner image is thermally fixed by the fixing unit 9 and is discharged onto the upper surface of the printer 1.

2. Configuration of High Voltage Power Supply Device Next, an electrical configuration related to the present invention of the printer 1 will be described with reference to FIG. FIG. 2 shows a schematic block diagram of a high-voltage power supply device 50 mounted on a circuit board (not shown) and a connection configuration related to the high-voltage power supply device 50.

The high-voltage power supply device 50 includes a CPU 51, a high-voltage power supply circuit 52 connected to the CPU 51, a ROM 53, a RAM 54, and a counter 55.
The CPU 51 controls the entire printer in addition to the control of the high-voltage power supply circuit 52. The counter 55 includes a printed sheet counter that counts up the number of sheets 15 to be printed, which is counted up each time a sheet detection signal output from the sheet sensor 37 is input. If the sheet 15 has not been printed normally, such as when a paper jam has occurred, the print number counter 55 counts down in consideration of the sheet 15 detected by the paper sensor 37 but not printed. You may do it. The ROM 53 stores various operation programs executed by the CPU 51, and the RAM 54 stores image data used for printing processing.

  The high-voltage power supply circuit 52 includes a charging voltage generation circuit (an example of a voltage application unit) 60, a grid voltage control circuit 71, a current detection circuit (an example of a current detection unit) 72, and a GRID control smoothing circuit 73. Here, the charging voltage generation circuit 60 is provided in common for each charger (41K to 41C), and includes a grid voltage control circuit (71K to 71C), a current detection circuit (72K to 72C), and a smooth for GRID control. The circuits (73K to 73C) are provided corresponding to the chargers (41K to 41C).

  The charging voltage generation circuit 60 includes a PWM smoothing circuit 61, a drive circuit 62, a booster circuit 63, and a voltage detection circuit (an example of a voltage detection unit) 68.

  The charging voltage generation circuit 60 generates a charging voltage CHG that is commonly applied to the discharge wires (42K to 42C) of the chargers (41K to 41C). Each grid voltage GRID is generated by the common charging voltage CHG and each grid voltage control circuit 71. The charging voltage CHG is, for example, 5.5 kV to 7 kV (positive polarity), and the grid voltage GRID is, for example, about 700 V (positive polarity).

  The PWM smoothing circuit 61 is composed of, for example, a resistor and a capacitor (not shown), smoothes a PWM (Pulse Width Modulation) signal from the port PWM1 of the CPU 51, and supplies the smoothed PWM signal to the drive circuit 62. To do.

  The drive circuit 62 is configured to cause an oscillation current to flow through the primary winding 64a of the transformer 64 of the booster circuit 63 based on, for example, the smoothed PWM signal received from the PWM smoothing circuit 62. Here, the value of the charging voltage CHG is controlled according to the duty ratio of the PWM signal. For example, the charging voltage CHG generated by the booster circuit 63 is controlled to increase as the duty ratio of the PWM signal increases.

  The booster circuit 63 includes, for example, a transformer 64, a rectifier diode 65, a smoothing capacitor 66, and an output resistor 67. The transformer 64 includes a primary side winding 64a, a secondary side winding 64b, and an auxiliary winding 64c.

  With such a configuration, the voltage of the primary winding 64a is boosted and rectified in the booster circuit 63, and is applied as the charging voltage CHG to the discharge wires (42K to 42C) of the chargers (41K to 41C).

  The voltage detection circuit 68 is connected between the auxiliary winding 64 c of the transformer 64 and the CPU 51. The voltage detection circuit 68 includes, for example, a rectifier diode D1, a smoothing capacitor C2, and voltage dividing resistors R2 and R3. The voltage detection circuit 68 detects the output voltage v1 generated between the auxiliary windings 64c according to the generation of the charging voltage CHG and supplies the detection signal D1 to the port A / D1.

  Each grid voltage control circuit 71 includes, for example, a resistor 81, a transistor Q, and a resistor 82 connected in series. Each grid voltage control circuit 71 adjusts the voltage between the collector and emitter of the transistor Q to control each grid voltage GRID to a predetermined value.

Each current detection circuit 72 includes a detection resistor 82 for detecting the grid current Ig, and detects the voltage of the detection resistor 82 to generate detection signals (D2K to D2C). Here, the configuration of each grid voltage control circuit 71 is used for each detection resistor 82. The detection signals (D2K to D2C) are supplied to the ports (A / D2 to A / D5) of the CPU 51, respectively. That is, in this embodiment, the charging current flowing through the photosensitive drum 44 is detected based on the GRID current Ig. This is because there is usually a certain correlation between the current flowing to the ground via the photosensitive drum 44 and the GRID current Ig, so that the magnitude of the current flowing to the photosensitive drum 44 can be estimated by the GRID current Ig. It is. In the charging voltage determination process described later, each current detection circuit 72 detects a current flowing from the charger 41 to the photosensitive drum 44 in a state where the photosensitive drum 44 is neutralized by the exposure device 45 (static elimination unit). To do.
The method of detecting the charging current is not limited to this, and for example, a detection resistor can be connected in series with the smoothing capacitor 66 of the charging voltage generation circuit 60 from the ground side and detected by the detection resistor.

  Similar to the PWM smoothing circuit 61, each GRID control smoothing circuit 73 is composed of, for example, a resistor and a capacitor (not shown), smooths the PWM signal from the ports (PWM2 to PWM5) of the CPU 51, and smoothes the smoothed PWM. The signal is supplied to the bases of the transistors (Q1 to Q4) of each grid voltage control circuit 71. The voltage between the collector and emitter of each transistor Q is adjusted by a smoothed PWM signal supplied to the base.

3. Printing Process Next, a printing process in the present embodiment will be described with reference to FIG. FIG. 3 is a flowchart showing each process related to the printing process. In the printing process, the target value change (charging voltage determination process) interval of the charging voltage CHG is changed according to the environmental humidity in which the printer 1 is installed. The reason for changing the target value change interval of the charging voltage CHG according to the environmental humidity is that the degree of contamination of the discharge wire 42 varies depending on the environmental humidity. The printing process is executed according to a predetermined program by a CPU (an example of a target value changing unit) 51 in accordance with a printing command.

  First, in step S205, the CPU 51 monitors the temperature and humidity in the printer 1 from the temperature and humidity sensor. Next, in step S210, it is determined whether the humidity is higher than a predetermined value A, for example, 75%. When it is determined that the humidity is higher than the predetermined value A (step S210: YES), the determination number, which is the number of sheets already printed, for determining whether or not to perform the charging voltage determination process is set to “500”. (Step S215).

  On the other hand, when it is determined that the humidity is not higher than the predetermined value A (step S210: NO), it is determined whether the humidity is lower than a predetermined value B, for example, 30% (step S220). When it is determined that the humidity is lower than the predetermined value B (step S220: YES), the determination number is set to “2000” (step S230). On the other hand, when it is determined that the humidity is not lower than the predetermined value B (step S220: NO), the determination number is set to “1000” (step S225). Here, the lower the humidity is, the larger the number of determinations is set. The lower the humidity, the less the contamination of the discharge wire 42 due to the discharge, so the number of prints that can be printed before the occurrence of abnormal discharge in the charger 41. This is because of the increase.

  Next, the CPU 51 determines whether or not the number of printed sheets from the previous charging voltage determination process is greater than or equal to the determined number based on the count number of the printed sheet counter 55 (step S240). If it is determined that the number of printed sheets is not equal to or greater than the determined number (step S240: NO), the process proceeds to step S255.

  On the other hand, if it is determined that the number of printed sheets is equal to or larger than the determined number (step S240: YES), it is determined that the discharge wire 42 is very dirty, the printed sheet counter 55 is reset in step S241, and in step S242. Then, the separation mechanism 35 (see FIG. 2) is controlled to separate each developing roller 47 from each photosensitive drum 44. That is, the CPU 51 places each photosensitive drum 44 and each developing roller 47 in a separated state when the surface of each removed photosensitive drum faces the developing roller 47.

  Here, the reason why each developing roller 47 is separated from each photosensitive drum 44 is to reliably prevent a large amount of developer from adhering to the static eliminating portion when the surface of each photosensitive member that has been neutralized faces each developing roller 47. It is to do. Thereafter, in step S245, a color target voltage value determination process described later is performed, and the color target voltage value is changed. In step S250, similarly, a monochrome target voltage value determination process, which will be described later, is performed, and the monochrome target voltage value is changed. As described above, in this embodiment, the target value of the charging voltage CHG corresponding to the printing mode is set, and an image can be formed with the optimum target value of the charging voltage CHG corresponding to the printing mode. Can be suppressed.

  Next, in step S255, it is determined whether the current printing is color mode printing. When the color mode printing is performed (step S255: YES), the printing process is executed with the color target voltage value determined in step S245 (step S265). On the other hand, if it is not color mode printing (step S255: NO), that is, if it is monochrome mode printing, printing processing is executed with the monochrome target voltage value determined in step S250 (step S260).

  If YES is determined in step S240, each developing roller 44 that has been separated from each photosensitive drum 44 during color mode printing cancels the separation operation by the separation mechanism 35, thereby again being applied to each photosensitive drum 44. Abutted. On the other hand, during monochrome mode printing, among the developing rollers 47 separated from the photosensitive drums 44, only the black (K) developing roller 47K similarly releases the separation operation by the separation mechanism 35, thereby again exposing the photosensitive drum 44. It is brought into contact with the drum 44K.

  Next, when all the printing processes related to the printing command are not completed (step S270: NO), the process returns to step S240, and when all the printing processes related to the printing command is completed (step S270: YES), this process is terminated. .

  As described above, when the humidity is high, the number of determinations is reduced, and the target value changing process interval of the charging voltage CHG is shortened. Therefore, it is possible to stably suppress a decrease in print image quality even at high humidity. Note that the interval of the target value changing process for the charging voltage CHG may be changed in consideration of the environmental temperature based on the temperature detected by the temperature / humidity sensor 36 regardless of the humidity. That is, the CPU 51 may shorten the interval of the target value changing process for the charging voltage CHG at high temperature and high humidity. Here, the time of high temperature and high humidity is, for example, a case where the temperature is 30 ° C. or higher and the humidity is 70% or higher.

  Further, the method of shortening the target value change processing interval of the charging voltage CHG is not limited to the number of determinations. For example, a timer for measuring the time from setting the target value of the charging voltage CHG is provided, and based on the measurement time of the timer The target value change processing interval may be shortened.

4). Charging Voltage Determination Processing Next, processing for determining the charging voltage (target voltage value) CHG in the present embodiment will be described with reference to FIGS. 4 and 5. FIG. 4 is a flowchart showing the color target voltage value determination process in step S245 of FIG. FIG. 5 is a flowchart showing the monochrome target voltage value determination process in step S250 of FIG. Each determination process is executed by a CPU (target value changing unit) 51 according to a predetermined program in accordance with a print command.

4-1. Color Target Voltage Value Determination Process In this process, the CPU 51 first sets the constant voltage target value of the charging voltage CHG to, for example, 6 kV in step S305 in FIG.

  Next, the CPU 51 performs constant voltage control on the charging voltage generation circuit 60 so that the charging voltage CHG becomes a target value based on the detection signal D1 (step S315).

  Subsequently, the exposure drums 45 expose all the colors (K, Y, M, C) of the photosensitive drums 44 to black (step S325). As described above, in this embodiment, before the optimum target voltage of the charging voltage CHG is determined, the surface of the photosensitive drum is neutralized by exposing the photosensitive drum 44 to black.

  Here, “black solid exposure” means that a predetermined surface area of the corresponding photosensitive drum 44 is uniformly exposed with a predetermined light intensity by the exposure device 45. Here, the predetermined surface area is, for example, the entire surface of the photosensitive drum 44. Here, the predetermined light intensity is, for example, the light intensity at the time of exposure during normal printing.

  As described above, by removing the charge of the photosensitive drum 44 by the exposure of the exposure device 45, a dedicated device is not required and the surface of the photosensitive member can be removed directly as compared with a charge removal method using an EL lamp or a transfer current.

  Next, in step S335, it is determined whether or not the charging voltage CHG has reached the target voltage. When it is determined that the charging voltage CHG has not reached the target voltage, the process returns to step S315, and when it is determined that the charging voltage CHG has reached the target voltage, the process proceeds to step S340.

  In step S340, the CPU 51 determines whether or not the GRID currents (Ig1 to Ig4) of the respective colors are equal to or greater than a predetermined current value based on the detection signals (D2K to D2C). Here, each current detection circuit 72 detects the current flowing to each photosensitive drum 44 in a state of being discharged by each exposure device 45 based on the GRID current (Ig1 to Ig4). When it is determined that the GRID current Ig is not equal to or greater than the predetermined current value, the process proceeds to step S350. Here, the predetermined current value is, for example, 250 μA.

  In step S350, the CPU 51 determines whether or not the charging voltage CHG is equal to or higher than a predetermined voltage value. Here, the predetermined voltage value is set to a value that suppresses the occurrence of abnormal discharge, for example, 7.5 kV. This can more reliably suppress the occurrence of abnormal discharge.

  When it is determined that the charging voltage CHG is equal to or higher than the predetermined voltage value (step S350: YES), a wire cleaning message is displayed on, for example, a liquid crystal display (not shown) of the operation panel to notify the wire cleaning (step S380). ). Thus, by notifying the user of the cleaning of the discharge wire, it is possible to improve the environment in which abnormal discharge is likely to occur. Then, this process ends.

  On the other hand, when the charging voltage CHG is not determined to be equal to or higher than the predetermined voltage value (step S350: NO), that is, when the charging voltage CHG is determined to be less than the predetermined voltage value, the target voltage is increased in order to increase the GRID current Ig. The value is increased by a predetermined voltage amount, for example, 200 V (step S355), and the process returns to step S315.

Note that the CPU 51 may increase the value of the predetermined voltage amount as the number of printed sheets (number of formed images) increases. In this case, since the target value of the charging voltage CHG can be set in consideration of the deterioration of the photosensitive drum 44, it is possible to suitably suppress the deterioration of the image quality.
Further, after the process of step S355, the process may be shifted to the process of step S345 without returning to the process of step S315. That is, the process of increasing the target value of the charging voltage CHG by a predetermined voltage amount (step S355) does not necessarily have to be repeated.

  If it is determined in step S340 that the GRID current Ig is greater than or equal to the predetermined current value, there is a difference (for example, 100 μA) that is greater than or equal to the predetermined value in the difference between the GRID currents (Ig1 to Ig4) of each color in step S345. Determine whether or not. If there is a difference greater than or equal to a predetermined value (step S345: YES), a wire cleaning message is notified that there is a possibility that the dirty discharge wire 42 exists (step S380). On the other hand, when the detected current value is not different from the predetermined value (step S345: NO), the smallest GRID current Ig is selected as the control target (step S360).

  Next, the constant current control value of the selection target GRID current Ig is set to 250 μA, for example (step S365). At this time, it is determined whether the charging voltage CHG is equal to or higher than the predetermined voltage value (step S370). If the charging voltage CHG is equal to or higher than the predetermined voltage value (step S370: YES), a wire cleaning message is notified (step S380).

  On the other hand, if the charging voltage CHG is not equal to or higher than the predetermined voltage value (step S370: NO), it is determined in step S375 whether or not the current value of the selected GRID current Ig is 250 μA. When the current value of the selected GRID current Ig is not 250 μA (step S375: NO), the process returns to step S365. On the other hand, if the current value of the selected GRID current Ig is 250 μA (step S375: YES), the value of the charging voltage CHG at this time is newly set as the color target voltage value for constant voltage control during color mode printing. (Step S385). Thus, the color target voltage value determination process is completed. Thereafter, when printing processing in the color mode is executed, an image is formed on the sheet 15 by the image forming unit 5 under constant voltage control of the charging voltage CHG with the changed color target voltage value.

4-2. Monochrome Target Voltage Value Determination Process Next, the monochrome target voltage value determination process will be described with reference to FIG. The same steps as those in the color target voltage value determination process in FIG. 3 are denoted by the same step numbers, the description thereof is omitted, and only differences from the color target voltage value determination process will be described.

  In step S315, the CPU 51 performs constant voltage control on the charging voltage generation circuit 60 so that the charging voltage CHG becomes a target value based on the detection signal D1, and then in step S325A, only the black color is applied by the black (K) exposure device 45. The photosensitive drum 44K is exposed to black.

Next, when it is determined in step S335 that the charging voltage CHG has reached the target voltage, the process proceeds to step S340. In step S340A, the CPU 51 determines that the black GRID current Ig1 is a predetermined current value based on the detection signal D2K. Judge whether it is above. If it is determined that the GRID current Ig1 is not equal to or greater than the predetermined current value, the process proceeds to step S350.
On the other hand, if it is determined in step S340A that the GRID current Ig1 is greater than or equal to the predetermined current value, the constant current control value of the GRID current Ig1 is set to, for example, 250 μA in step S365A.

  Next, in step S370, it is determined that the charging voltage CHG is not equal to or higher than the predetermined voltage value. In step S375A, it is determined whether the current value of the GRID current Ig1 is 250 μA. When the current value of the GRID current Ig1 is 250 μA (step S375A: YES), the value of the charging voltage CHG at this time is newly set as a monochrome target voltage value for constant voltage control during monochrome mode printing (step S385A). ). This completes the monochrome target voltage value determination process. Thereafter, when printing processing in the monochrome mode is executed, an image is formed on the sheet 15 by the image forming unit 5 under constant voltage control of the charging voltage CHG with the changed monochrome target voltage value.

5). Effects of Embodiment The charging current flowing through the photosensitive drum 44 that has been neutralized is detected based on the GRID current Ig, and is determined based on the detected current, that is, based on the substantially maximum current required for charging the photosensitive drum 44. Voltage control can be performed. Therefore, even if black solid printing (one-side black printing) is performed, in which the occurrence of abnormal discharge is suppressed and the charging potential of the photosensitive drum 44 is reduced to the maximum, the photosensitive drum 44 can be sufficiently charged thereafter. Can be suppressed.

  At that time, the CPU 51 repeats the process of increasing the target value of the charging voltage CHG by a predetermined voltage amount (200 V) until the detected current becomes equal to or higher than the predetermined current value (250 μA). For this reason, the target value of the charging voltage that can sufficiently charge the photosensitive drum 44 can be set with certainty, so that it is possible to reliably suppress deterioration in image quality.

  Further, the current detection circuit 72 detects the current flowing through the photosensitive drum 44 by detecting the current flowing through the grid 43. Since it is not necessary to directly detect the current flowing to the photosensitive drum 44, deterioration of the surface of the photosensitive member can be suppressed. Alternatively, a small detection resistor having a low withstand voltage can be used as compared with the case where the charging current is detected directly from the charging voltage CHG by resistance.

  In the monochrome mode, only the photosensitive drum 44K corresponding to black is discharged, and in the color mode, the photosensitive drum 44 corresponding to each color is discharged. For this reason, when a plurality of chargers 41 are commonly controlled by a single charging voltage generation circuit 60, it is sufficient to set the target value by discharging only the photosensitive drum 44 to be used depending on the mode. Can be reduced.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention.

  (1) In the above embodiment, an example in which the predetermined surface area of the photosensitive drum 44 to be neutralized is the entire surface of the photosensitive drum 44 is shown, but the present invention is not limited thereto. For example, when the surface of the photosensitive drum 44 is partially neutralized and the CPU 51 detects that the current detected when the neutralized surface of the photosensitive drum 44 faces the charger 41 is less than a predetermined current value, the charging voltage CHG The target value may be increased by a predetermined voltage amount. In this case, for example, the non-image forming area in the longitudinal direction (axial direction) of the photosensitive drum 44 is not neutralized, and only the image forming area is neutralized, so that the photosensitive drum 44 can be sufficiently charged, and An excessive current can be suppressed from flowing.

  (2) In the above embodiment, the example in which the present invention is applied to the printer 1 in which the charging voltage generation circuit 60 is provided in common to each charger 41 as shown in FIG. Not limited to. The present invention can also be applied to a printer in which the charging voltage generation circuit 60 is provided for each charger 41 of each color. At that time, the above-described constant voltage control of the charging voltage CHG is performed on each charging voltage generation circuit 60. At this time, for example, step S345 and step S360 of FIG. 4 are omitted.

  When the charging voltage generating circuit 60 is not shared as described above, all the photosensitive drums 44 can be charged in the color mode, and only the black photosensitive drum 44K can be charged in the monochrome mode. be able to. In this case, the degree of contamination of the discharge wires other than black (42Y, 42M, 42C) varies depending on the frequency of the color mode. Therefore, it is preferable to provide a counter 55A that counts only during color mode printing and a counter 55B that counts in both color mode and monochrome mode as the print number counter 55. Then, if the value of the counter 55A is equal to or greater than the determination number of sheets, processing for determining the respective charging target voltage values for Y, M, and C is performed, and if the value of the counter 55B is equal to or greater than the determination number of sheets, What is necessary is just to perform the process which determines a value.

DESCRIPTION OF SYMBOLS 1 ... Printer 5 ... Image formation part 41 ... Charger 42 ... Discharge wire 43 ... Grid 44 ... Photosensitive drum 51 ... CPU
60 ... Charge voltage generation circuit 68 ... Voltage detection circuit 72 ... Current detection circuit

Claims (13)

A photoreceptor,
A charger having a discharge wire and a grid and charging the photoreceptor;
A voltage applying unit that generates a charging voltage and applies the generated charging voltage to the charger;
A voltage detector for detecting the charging voltage;
Based on the detection voltage detected by the voltage detection unit, a control unit for constant voltage control of the voltage application unit,
A static elimination unit for neutralizing the photoconductor;
A current detection unit for detecting a current flowing from the charger to the photoconductor in a state of being neutralized by the neutralization unit;
A target value changing unit that increases the target value of the charging voltage for the constant voltage control by the control unit when the detected current detected by the current detection unit is less than a predetermined current value;
An image forming unit that forms an image on a recording medium under constant voltage control of the charging voltage at the target value changed by the target value changing unit;
An image forming apparatus. The image forming apparatus according to claim 1.
The target value changing unit repeats the process of increasing the target value of the charging voltage by the predetermined voltage amount until the detected current becomes equal to or greater than the predetermined current value. The image forming apparatus according to claim 1, wherein:
The image forming apparatus, wherein the current detection unit detects a current flowing through the photosensitive member by detecting a current flowing through the grid of the charger. The image forming apparatus according to any one of claims 1 to 3,
The static eliminating portion is for partially neutralizing the surface of the photoconductor,
The target value changing unit increases the target value of the charging voltage by the predetermined voltage amount when the current detected when the surface portion of the photosensitive member that has been neutralized faces the charger, is less than the predetermined current value. An image forming apparatus. The image forming apparatus according to claim 1, wherein:
When the detected current is less than the predetermined current value and the charging voltage detected by the voltage detecting unit is less than the predetermined voltage value, the target changing unit sets the target value of the charging voltage to the predetermined voltage amount. Increase the image forming apparatus. The image forming apparatus according to claim 5.
The image forming apparatus, wherein the control unit notifies cleaning of the discharge wire if the charging voltage detected by the voltage detection unit is equal to or higher than the predetermined voltage value. The image forming apparatus according to any one of claims 1 to 6,
An exposure unit that forms a charged latent image on the surface of the photosensitive member by exposing the photosensitive member;
An image forming apparatus in which the exposure unit constitutes the charge eliminating unit by uniformly exposing a predetermined surface area of the photoconductor with a predetermined light intensity by the exposure unit. The image forming apparatus according to claim 7.
A developing unit for developing a charged latent image on the surface of the photoreceptor;
The control unit is an image forming apparatus in which the photosensitive member and the developing device are separated from each other when the surface of the discharged photosensitive member faces the developing device. The image forming apparatus according to any one of claims 1 to 8,
A plurality of sets of the photoreceptor and the charger corresponding to each printing color are provided,
The image forming apparatus, wherein the voltage application unit applies the charging voltage to a plurality of chargers in common. The image forming apparatus according to claim 9, wherein the image forming apparatus includes:
It has a printing mode of a monochrome mode in which printing is performed using a black developer and a color mode in which printing is performed using a plurality of color developers,
The target value changing unit sets a monochrome target voltage value of the charging voltage in the monochrome mode and a color target voltage value of the charging voltage in the color mode,
The neutralization unit neutralizes only the photosensitive member corresponding to black when the monochrome target voltage value is set, and neutralizes the photosensitive member corresponding to each color when setting the color target voltage value,
The image forming apparatus forms an image on a recording medium under constant voltage control with a target value of the charging voltage corresponding to a mode. The image forming apparatus according to any one of claims 1 to 10,
The target value changing unit increases the value of the predetermined voltage amount as the number of formed images increases. The image forming apparatus according to any one of claims 1 to 11,
The target value changing unit is an image forming apparatus that shortens an interval of a target value changing process of a charging voltage at high temperature and high humidity. In an image forming apparatus comprising: a photoconductor; a charger that charges the photoconductor with a discharge wire and a grid; and a voltage application unit that applies a constant voltage controlled charging voltage to the charger. A method of forming,
Removing the charge of the photoreceptor;
Applying a predetermined voltage from the voltage application unit to the charger, and detecting a current flowing from the charger to the photoconductor in a neutralized state;
When the detected current detected is less than a predetermined current value, increasing the target value of the charging voltage for constant voltage control by a predetermined voltage amount; and
Forming an image on a recording medium under constant voltage control of the charging voltage at the changed target value;
An image forming method comprising:

Images